Chapter 7 Lab Exercise: Mode Conversion Due to Changes in Electrical Balance

Common-mode current on cables is a major source of conducted and radiated emissions. Differential-mode signals can be converted to common-mode noise when their path of propagation experiences a change in electrical balance.

Preparation:  Students need to know become familiar with the definitions of differential-mode and common-mode as described in Chapter 7.

Equipment Required:

• Vector network analyzer
• two sections of coaxial cable (30-100 cm)
• two sections of twisted or untwisted wire pair cable (30-100 cm)
• RF current probe (30-100 MHz)
• balun or isolation transformer that works up to 100 MHz.
• 50 and 100-ohm cable terminations
• snap-on ferrite cores (optional)

Procedure:

Step 1: Connect Port 1 of the VNA to one section of the coaxial cable. Connect the other end of the cable to the second section of coaxial cable. Terminate the second section with a matched (50-Ω) termination resistance.

Step 2: Connect Port 2 to the RF current probe and place the probe around the first section of coaxial cable at a fixed and repeatable position. Record the peak amplitude and frequency of the measured common-mode current between 30 and 100 MHz. (This will be a very small value, perhaps below the noise floor. Reducing the resolution bandwidth and video bandwidth may lower the noise floor sufficiently to detect this current.)

Step 3: Replace the second section of the coaxial cable with a section of wire-pair cable. The replacement should have the same length as the coaxial cable but be terminated with a matched (~100-Ω) resistance. Record the peak amplitude and frequency of the measured common-mode current between 30 and 100 MHz. (The imbalance change at the connection between the two cable sections creates a common-mode voltage that drives one cable section relative to the other. A significant common-mode current should be detected.)

Step 4: Place the balun on the Port 1 output of the VNA and use it to drive a section of the wire-pair cable. Connect the other end of the cable to the second section of wire-pair cable. Terminate the second section with a matched (~100-Ω) resistance. 

Step 5: Connect Port 2 to the RF current probe and place the probe around the first section of wire-pair cable at a fixed and repeatable position. Record the peak amplitude and frequency of the measured common-mode current between 30 and 100 MHz. (This will be a small value.)

Step 6: Replace the second section of the wire-pair cable with a section of coaxial cable. The replacement should have the same length but be terminated with a matched (50-Ω) resistance. Record the peak amplitude and frequency of the measured common-mode current between 30 and 100 MHz. (The imbalance change at the connection between the two cable sections creates a common-mode voltage that drives one cable section relative to the other. A significant common-mode current should be detected.)

Step 7: Try clamping a snap-on ferrite core around the cable at various positions. How much reduction in the common-mode current can be achieved this way?

Notes: 

The common-mode current is proportional to the differential-mode voltage at the connection as well as the change in the imbalance. Both the balun and the different termination impedances alter this voltage between test set-ups. This voltage difference is easily calculated and can be accounted for when comparing the common-mode currents in the different test configurations. 

The common-mode current in the coaxial cable configuration can be difficult to measure if all of the connectors are shielded coaxial connections. In this case, there are no external fields so the level of imbalance is nearly perfect in every location. On the other hand, achieving perfect balance in a differential signal path is more difficult. Nevertheless, the common-mode currents generated when everything is nominally balanced should be much lower than those observed when there is a clear change in the electrical balance.